Evolutionary fitness dictates that threat must be detected quickly. Rapid identification of potential harm has resulted in a relatively rapid, automatic threat detection system with a bias toward false positives. Given its well-documented role in fear and responses to salient stimuli, the amygdala has been identified as a likely facilitator of rapid detection of potential threats. Following work indicating that specific phobics respond more rapidly to phobia-relevant stimuli, Larson and colleagues (2002) found more rapid amygdala activation among specific phobics during phobia-relevant compared to neutral and other negative stimuli, and compared to nonphobic's responses to phobia-relevant stimuli. These data underscore the need to consider the dynamic process of emotion and its neural instantiation to more fully characterize healthy and pathological affective responses. While the aforementioned data point to rapid amygdala responding as a potential characteristic defining phobic fear, it is also possible that flexibility in the time course of the amygdala response is a normal, adaptive process such that during initial presentations of salient affective stimuli the amygdala responds more rapidly to prepare for potential threat. Over time and decreasing likelihood of threat this response slows. To test the idea that the chronometry of amygdala activation is flexible with respect to stimulus salience, 15 healthy subjects will complete an fMRI investigation of the time course of amygdala responding during an aversive conditioning paradigm. It is hypothesized that amygdale responses will be more rapid early in the conditioning phase compared to late conditioning and extinction periods. In addition, the dynamic pattern of amygdala activity during two means of attenuating responses to aversively conditioned stimuli, extinction and conditioned inhibition, will be compared. Thus, the major aim of the proposed study is to further examine the dynamic process of the neural substrates of emotional experience, including both time course and magnitude of the neural instantiation of affective processes.